Shown off over the weekend at PDF: Applied, Seetra.in is just a prototype web app for now. Though it works on both the iOS and Android devices, you're going to need an internet connection to use it and you can only get that in a few stations. It just works on the 1,2, and 3 line, which runs along the west side of Manhattan. So you'll have to remember to use it before you go underground and when you happen to be in that part of town.
Though limited at this point, Seetra.in destined to expand its coverage, especially considering the Metropolitan Transit Authority (MTA), the body that governs public transit in NYC, is continuing its efforts to make train data publicly available. On top of that, this is something a lot of people want, myself among them. (Literally, I was complaining about this three days ago). Until then, proceed as usual with your public transit frustrations. [FastCo]